1. Field of the Invention
The invention relates to apparatus for coating moving webs and more particularly relates to such apparatus including improved means for recirculating excesses of coatings, initially applied to the web.
2. Brief Description of the Prior Art
The prior art is replete with descriptions of apparatus for the continuous coating of moving webs such as continuous lengths of paper sheet materials. In one type of apparatus, the coating liquid is brought to the moving web by an applicator roller which is partially immersed in a reservoir of the coating liquid. Excesses of the applied coating are doctored off the web with a mechanical scraper or an airknife. The doctored excess is directed back into the applicator reservoir where it mixes with fresh or make-up coating liquid. This arrangement was not satisfactory for all purposes, since the applied coating composition generally undergoes compositional changes while in contact with the web. The doctored excess returned to the coating application reservoir may have a different compositional make-up than the unapplied coating materials. It wil be appreciated that incomplete mixing of the two, essentially different coating materials, generally results in coating marks becoming highly visible on the coated web due to the heterogenicity of the actually applied coating compositions.
In order to solve the aforementioned problem in the prior art, it has been proposed to add pumps to circulate and recirculate the liquids, between the coating reservoir and a separate collection vessel adapted to receive the doctored excess coating. This improves mixing and homogenicity of the coating composition being applied and retained on the web. However, the introduction of pumps is not an entirely satisfactory solution to the prior art problems. Not only does the addition of a pump recirculating system represent a significant capital investment, but pumps tend to generate foams in some coating liquids. Pumps tend to introduce air into the coating liquids, through defective gaskets or from too rapid linear flow speeds.
The present invention is an improvement over the prior art apparatus in that efficient mixing and recirculation of the doctored excesses of coating liquids is achieved without the use or need for external pumps. Instead, by repositioning of certain apparatus components an "innate pumping" system is obtained, which does not generate foams. Energy is conserved employing the improved apparatus of the invention and a high level of coating quality on the finished web is achieved.